Rammed earth is, as its name suggests, an earth mixture which is pounded or rammed into layers within box-like formwork to form a wall. Once the formwork is removed, a layer cake effect is wrought on the exposed earth wall. Although the wall will continue to gain strength over a long period of time, it is almost immediately structurally sound.
Modern rammed earth walls tend to have a minimum thickness of 300 mm, although 150 mm to 200 mm may be used internally. The best earth mix will have a clay content less than 50 per cent, sometimes as low as 10-15 per cent. It normally has a reasonably high degree of fine and course sand — up to 80 per cent.
Where cement or lime is added, this is rarely more than 10 per cent, as this can affect the walls’ ability to absorb moisture from the air (hygroscopicity), which can limit its ability as a finished building to ‘breathe’. The mix needs to have just enough moisture that it binds together like, well, a cake mix.
Ancient History
By some accounts, rammed earth dates back well over 7,000 years. As is often noted, some sections of the famed Great Wall of China were constructed with this method.
One method superior to others, and which was known to the Romans, has been preserved by tradition to modern times. This method consists of ramming slightly moist, specially selected earth without the addition of straw or other material between movable forms and is known by its French name, “pisé de terre”, which means ‘rammed earth’.
Australia became a hotbed of rammed earth development in the late 70s and 80s. Pioneers like Steve Dobson, director of Ramtec, and Giles Hohnen of Earth House, were filling up south WA with everything, from churches to wineries, with the method.
Our own leading lights of architectural design have also added rammed earth to their portfolios. In partnerships with other cutting edge firms, both Glenn Murcutt and Philip Cox have their names attached, respectively, to the Bowali Visitor Information Centre in Kakadu National Park, Northern Territory, and the National Wine Centre in Adelaide. And Gregory Burgess Architects has been winning awards for its rammed earth public buildings from the cliff tops above the Twelve Apostles to the Victorian High Country.
More recently, architect James Stockwell designed a rammed earth house in the Blue Mountains which won the 2008 top residential award for new housing, Royal Australia Institute of Architects Wilkinson Award for Residential Architecture.
Eco attributes
What are the environmental benefits of building with rammed earth? Firstly there is the material. It is generally abundant and can often be sourced locally, although stress and shrink testing is required to ensure that on-site earth is equal to the reliable consistency found in say, road base, a commonly used rammed earth material.
In 2002, Canada’s national housing agency, the Canada Mortgage and Housing Corporation, released a report entitled Alternative Wall Systems for Low-rise Housing. It referenced a report comparing the material efficiency of timber houses and earth houses: “Five acres of land can only provide enough lumber for 20 houses, whereas a 5 acre pit can provide enough earth for the walls of 5,000 homes.”
Some reports suggest rammed earth has been calculated as using 1/700th of the energy in the ramming process, compared to the energy used firing clay bricks of equal weight. Such estimates largely result from the fact that the materials used are not fired under intensive heat, although it should be noted that the roughly 6-10 per cent cement additive is created in a kiln process. In Building Materials Energy and the Environment, Bill Lawson tables the embodied energy of stabilised earth at 0.7 MegaJoules per kilogram (MJ/kg), listing clay bricks at 2.5 MJ/kg, close to four times higher.
A rammed earth building that forms part of the Environmental Technology Centre at Murdoch University in Western Australia actually used recycled building rubble mostly in the form of red bricks, which also lent the building a distinctive colour. The much referenced 100-room Kooralbyn Hotel Resort is also made of rammed earth. However, had cement bagged brickwork been selected, the embodied energy would have increased by 65 per cent.
Energy efficiency in use is another characteristic of rammed earth. The super thick walls, although offering minimal insulation, in pure terms, do provide substantial thermal mass. The walls have a lag of about 12 hours before they release the heat or ‘coolth’ previously absorbed from the inner and outer atmosphere.
Dobson says: “The R-value of walls may be described as resistive insulation. The thermal mass may be described as capacitive insulation. These are electrical analogies and are likening wall properties to an electrical resistor which slows/lessens the flow of electricity/energy, and a capacitor which stores energy and then releases it at a later time.”
Having such mass, the walls of a rammed earth building are very long lasting. Durability, or the absence of obsolence, is a key tenement of eco- design. And buildings like the rammed earth barn near Oberon, NSW, which was built in 1827 and still stands, are testament to this longevity. Painting and subsequent repainting is also not required as the wall’s colour goes all the way through.
There are obvious tangible benefits like termite resistance, substantial sound proofing, and in light of the recent disaster in Victoria, a significant degree of fire resistance. Dobson says a similar load bearing 250 mm thick adobe block wall achieved a fire resistance rating of four hours in a test by the National Building Technology Centre at Ryde in Sydney, well exceeding the half-hour rating required for an Extreme Zone 6 house. Dobson also recounts stories of rammed earth houses that have withstood category five cyclones that took apart 80 year-old homesteads in north-west Australia.
But probably the most lasting impression most people take away from visiting a rammed earth construction is not so much its technical attributes, but the ambience it evokes. Like the grain in quality timber, rammed earth walls invite touch. Aside from its obvious natural beauty, there is a distinct sense of strength and security that emanates from its sandy hued bulk.
Reality bites
There are a few concerns that rammed earth has had to counter on its journey to slowly becoming a more mainstream modern building material. For example, because the walls are so thick there is some reduction in the usable internal space for the same footprint of, say, a brick veneer structure. By virtue of being a relatively niche construction method, there are also fewer skilled practitioners with knowledge in working with rammed earth. On the plus side, the few there are tend to be very experienced in their tradecraft.
Environ wrote back in 2003 about the rammed earth off-the-grid house of Caroline Cook and her husband Dean. We contacted Dean again to glean any thoughts he had after six or so years in residence. A qualified bricklayer, he’d worked alongside Peter deJong of Earthright to build his own three- bedroom, split-level, rammed earth house.
“Well, I’d probably go with less thermal mass internally,” he says. Aside from a feature wall of recycled brick, all his internal walls are rammed earth. “It’s brilliant in winter — we hardly ever have the heating on. But in summer, with many consecutive hot, dusty days, we often don’t get the opportunity to release the heat that has progressively accumulated in the walls.
“Because you can’t easily retrofit afterwards, we might’ve planned for more sockets for power, internet, etc, so we could more easily move room furniture around. And maybe designed in more reflective surfaces, like a few lime- washed walls.” This is because Dean says having all the walls a soft, sandy colour, instead of the usual white plasterboard, means you become accustomed to lower levels of light.
Conversely though, he loves that the walls are low maintenance with no painting. Six years ago he applied a silicon surface spray to the exterior walls and the bathroom, with a water-based spray to interior walls. He hasn’t needed to revisit it since. The spray allows for the walls to breath while reducing dust. Dean also appreciates that the solid walls mean anything can be hung up by simply inserting a masonry bolt.
Australia is blessed with a wealth of rammed earth buildings, ranging from the high profile projects like the National Wine Centre, which boasts the ‘largest rammed earth wall in the Southern Hemisphere’, to the much more modest, but equally impressive, rammed earth public toilets at Uluru.
Yet for all this interest, the general public is largely unaware of the eco and aesthetic wonder that is rammed earth. With over 700 rammed earth projects under his belt, it’s easy to see why Dobson wants to see greater uptake of the medium. He is of the view that rammed earth “is an environmental material whose time has come”, and faced with the twin accelerating dilemmas of peak oil and climate change, now is maybe that moment.
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